1. Field of the invention
The present invention relates to a method by which a mobile device including an RNSS satellite radio navigation receiver acquires satellite data.
2. Description of the prior art
In the field of mobile telephony, it is proving increasingly necessary to be able to locate mobile telephones.
To this end, it is known in the art to associate a cellular radio telephony device, for example of the Global System for Mobile communications (GSM) mobile telephone type, with a Radio Navigation Satellite System (RNSS) receiver such as a Global Positioning System (GPS), GLONASS or GALILEO type receiver by means of which the mobile device picks up transmissions from satellites to determine its position. Thus in the event of a road traffic accident, for example, the mobile device can calculate and transmit its position.
The position of the device may be determined in the following manner: a plurality of satellites transmit continuously a time-stamped signal that is picked up by the receiver. If it is synchronized to the clock of the satellites, the receiver can then measure the propagation time of this signal and deduce therefrom a distance between it and a particular satellite. Using three satellites, a receiver of the above kind can determine its position by triangulation. Each propagation time measurement represents the radius of a sphere centered on a particular satellite, the receiver being situated on that sphere. With two distance measurements, the position of a receiver is within a circle formed by the intersection of two spheres. A simultaneous third measurement reduces the intersection to two points, one of which is at a great distance away in space and is easily ignored.
In a satellite positioning system using RNSS type receivers, the data signals enabling the position of the receiver to be calculated come from different satellites (a minimum of four satellites to determine four unknowns x, y, z and t).
The GPS signal transmitted by each of the satellites is based on a spread spectrum technique. Thus the signal is a binary data signal modulated by a signal whose spectrum has been spread using a code division multiple access (CDMA) technique. In other words, each bit of the data signal is replaced by a spreading sequence specific to each satellite. The data is transmitted in serial mode at 50 bps (0.02 s/bit). A spreading sequence such as a Gold type pseudo-random sequence is transmitted at a much higher rate: a Gold sequence may be considered as a series of bits clocked at a clearly defined clock period; the expression “code moment” or the more frequently encountered term “chip” designates a bit of the sequence and, by extension, its duration. The spreading sequence is therefore transmitted at a rate of 1.023 Mchip/s (the duration of a chip is therefore approximately 1 μs) and includes 1023 chips (i.e. it has a duration of 1 ms); there are therefore 20 sequence repetitions per data bit.
As a result of modulation by the spread spectrum signal, a standard demodulator sees the received signal as noise.
As a general rule, the correlation function f(τ) of two signals fi(t) and fj(t) is given by the equation
            f      ⁡              (        τ        )              =                  ∫                  +          ∞                          -          ∞                    ⁢                                    f            i                    ⁡                      (            t            )                          ·                              f            j                    ⁡                      (                          t              -              τ                        )                          ·                  ⅆ          t                      ,in which τ designates a variable time. Of course, in practice, the integration is not effected from −∞ to +∞, but over a finite time period, the integral being divided by the duration of said period. The expression “autocorrelation” function is used if the functions fi(t) and fj(t) are identical and the expression “intercorrelation function” is used if the functions fi(t) and fj(t) are different.
Each satellite k has its own pseudo-random signal ck(t). Each of these pseudo-random signals has the property that its autocorrelation function is null except in the vicinity of the null time shift, where it assumes a triangular shape; in other words, the integral
      ∫          +      ∞              -      ∞        ⁢                    c        k            ⁡              (        t        )              ·                  c        k            ⁡              (                  t          -          τ                )              ·          ⅆ      t      is null when τ is non-null and is at a maximum when τ is null.
Furthermore, the signals associated with different satellites are selected so that their intercorrelation function is null; in other words, the integral
      ∫          +      ∞              -      ∞        ⁢                    c        k            ⁡              (        t        )              ·                  c        k            ⁡              (                  t          -          τ                )              ·          ⅆ      t      is null regardless of the value of τ when k and k′ are different.
The spread spectrum signals from the satellites are therefore selected to be orthogonal.
When the receiver is seeking to acquire data from a particular satellite, it correlates the received signal with a duplicate of the pseudo-random sequence of the satellite it is looking for (the sequence of the satellite is assigned to it once and for all and does not change during the service life of the satellite).
The received signal S(t) is therefore the sum of all the signals transmitted by each satellite:
            S      ⁡              (        t        )              =                  ∑                  k          =          1                n            ⁢                                    c            k                    ⁡                      (            t            )                          ·                              d            k                    ⁡                      (            t            )                                ,where n is the number of satellites, ck(t) is the spread spectrum signal from the satellite k and dk(t) is the data from the satellite k.
When seeking to acquire data from the satellite m, the local duplicate corresponds to the signal cm(t). Accordingly, after correlation, and assuming that the spread spectrum signals are perfectly orthogonal, all the data from the satellites other than the one that is being looked for (for which the intercorrelation functions are null) is eliminated, so that only data from the satellite m is retained. Correlation is possible because the duration of a spreading sequence is one twentieth the duration of a data bit.
The signal acquisition phase therefore consists in calculating the correlation of the received signal with the local duplicate of the satellite code that is being looked for, over a time period equal to the period of the code, which is 1 ms, and with a depth (bound of the integral) depending on the required detection performance. The receiver delays the start of the duplicate to obtain a triangular correlation peak. The value of this delay is therefore the time taken by the signal to propagate from the satellite to the user. This kind of measurement demands extremely high accuracy (better than 100 nanoseconds). The time taken by the signal to travel this distance is of the order of 100 milliseconds. However, because the clock of the GPS receiver is never fully synchronized to that of the satellites, the receiver has to adjust its clock constantly by a process of successive approximations to arrive at the maximum correlation of the two signals. Acquisition of the signal therefore necessitates a time sweep by the receiver.
Furthermore, the signal transmitted by each satellite is transmitted at a known frequency of 1575.42 MHz. The Doppler effect of the satellite, on which is superimposed a receiver local clock uncertainty, results in an uncertainty of ±5 kHz in respect of the signal received by the GPS receiver. Now, to obtain a good correlation, the signal generated locally by the receiver must have the same frequency as the signal transmitted by the satellite. Thus, in addition to the time sweep, the receiver must perform a frequency sweep in order to determine the time taken by the signal to propagate from the satellite to the user.
The time and frequency sweeps referred to above imply a very long data processing time and entail a receiver having a very high computation power.
One solution is to use a server to assist the GPS receiver of the mobile device by increasing its sensitivity by reducing the time-frequency area to be swept. A server of this kind is described in “Indoor GPS Technology” (F. van Diggelen et al., CTIA Wireless-Agenda, Dallas, May 2001). This technology is known as the assisted GPS (A-GPS) technology. FIG. 1 represents a telecommunication system 1 using an assistance server 5 of this kind. A mobile device 2 including a GPS receiver, such as a mobile telephone of a GSM type telephone network 4, is seeking to calculate its position from signals P1 to P4 transmitted by at least one of the satellites S1 to S4. To this end, the device 2 sends a request R in the form of a radio signal over the telephone network 4. The request R passes through a base transceiver state (BTS) type radio base station 3 associated with the cell in which the mobile device 2 is located. The request R is processed by the server 5, which receives satellite information in real time via fixed radio stations 6 equipped with GPS receivers receiving information K. In response to the request R, the server 5 sends to the mobile device 2 information I that passes through the BTS 3. The information contains, for example, the ephemerides of the satellites S1 to S4. Using that information, the mobile device 2 can determine the Doppler effect of the satellites and considerably curtail its frequency sweep. Note that there are two types of A-GPS technology, namely mobile station based (MS-based) and mobile station assisted (MS-assisted). In the case of the MS-based technology, the position of the mobile device 2 is calculated by the mobile device itself. In the case of the MS-assisted technology, the position of the mobile device 2 is calculated by the server 5.
Certain difficulties remain when using this kind of solution, however. In fact, one of the drawbacks of the above solution lies in the acquisition of data successively and independently for each satellite (determination of the signal propagation time, usually referred to as navigation signal acquisition); this kind of acquisition leads to a very long data processing time.
The present invention aims to provide a faster method for acquisition of satellite data by a mobile device including an RNSS satellite radio navigation receiver.